1. Field of the Invention
This invention relates to axial flow rotary machines, and more particularly to outer air seals circumscribing the blade tips of a rotor stage.
2. Description of the Prior Art
Gas turbine engines are illustrative of rotary machines in which the inventive concepts may be employed. In a gas turbine engine working medium gases are compressed by a first series of rotor mounted blades in the compression section and are flowed axially downstream to a combustion section. Fuel is combined with the compressed gases and burned in the combustion section to add thermal energy to the flowing medium. Downstream of the combustion section the gases are flowed across a second series of rotor mounted blades in the turbine section. Energy extracted from the medium by the second series of rotor blades is utilized to drive the blades of the compression section. The blades of both the compression and turbine sections are arranged in stages, or rows. The tips of the blades in each row are circumscribed by an outer air seal.
Aerodynamic performance within the engine is largely dependent upon the radial clearance between the outer air seal and the circumscribed blade tips. Even the slightest clearance degrades performance as the working medium gases escape over the tips of the blades. The problem is particularly severe in high temperature machines having radical thermal fluctuations over the operating range of the machine. The blades of the rotor respond immediately to changes in the temperature of the working medium gases. The conventional outer air seal responds much more slowly to these changes. Substantial initial clearances are provided between the tips of the rotor blades and the outer air seals to protect the respective structures from destructive interference during transient thermal conditions.
As the machine reaches thermal equilibrium the outer air seals tend to grow away from the rotor blade tips to a clearance which is on the same order of magnitude as the initial clearance discussed above. Past efforts for reducing clearances have primarily included elaborate case cooling systems for reducing the steady state temperature of the structure supporting the outer air seal. Reducing the temperature of the supporting structure limits the outward radial growth of the seal beyond the diameter required for transient clearance. An effective reduction in tip clearance results, although not without a substantial diversion of the working medium gases for case cooling. U.S. Pat. No. 3,583,824 to Smuland et al entitled "Temperature Controlled Shroud and Shroud Support" is representative of outer air seal structures employing cooling techniques.
Other approaches to reduced seal clearance include that shown in U.S. Pat. No. 2,488,875 to Morley entitled "Gas Turbine Engine". In Morley the vanes of the stator stage engage an inner case structure at a spigot and socket arrangement. The outer air seals extend axially from the stator vanes to circumscribe the tips of the rotor blades.
Although solutions proposed in the past for reducing blade tip clearance have been partially successful, scientists and engineers in the gas turbine art are continuing to search for structures offering improved engine performance.